It has generally been accepted that the nitrogen oxides (abbreviated hereafter as NO.sub.x) in combustion waste gas consist mainly of nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2). Since NO.sub.2 forms to some extent nitric acid, nitrous acid or salts thereof when absorbed in water or aqueous alkaline solution, some removal methods have been proposed based on this property.
On the other hand, there is little development in the removal of NO, because NO is not readily absorbed in water or alkaline solution. As shown in the following equation: EQU 2NO + O.sub.2 .fwdarw. 2NO.sub.2
the oxidation efficiency of NO is proportional to the square of the NO concentration and, for this reason, the oxidation of NO contained in industrial waste gas is in such low concentration as the order of ppm, which makes its removal very difficult.
Although the use of an adsorption agent, such as activated charcoal, active alumina and zeolite has been proposed for eliminating NO.sub.x, the adsorption efficiency of these adsorption agents is low, and at the same time their regeneration by wet methods is extremely difficult. Consequently NO.sub.x is usually discharged by dry methods through heating of the adsorption agent at a high temperature. When activated charcoal is employed as an adsorption agent, it must be heated up to the temperature of 200.degree. to 300.degree.C to discharge the NO.sub.x, resulting in a red heat state. Deterioration and consumption of the charcoal are therefore inevitable and, furthermore, explosion hazards exist. For these reasons, activated charcoal has not been utilized to any extent as an adsorption agent for this purpose.